diff --git a/GNUmakefile b/GNUmakefile
index fa06ff2d48..67036ebe56 100644
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -220,7 +220,7 @@ vm/resources.o:
 	$(TOOLCHAIN_PREFIX)$(WINDRES) vm/factor.rs vm/resources.o
 
 vm/ffi_test.o: vm/ffi_test.c
-	$(TOOLCHAIN_PREFIX)$(CC) -c $(CFLAGS) $(FFI_TEST_CFLAGS) -o $@ $<
+	$(TOOLCHAIN_PREFIX)$(CC) -c $(CFLAGS) $(FFI_TEST_CFLAGS) -std=c99 -o $@ $<
 
 vm/master.hpp.gch: vm/master.hpp $(MASTER_HEADERS)
 	$(TOOLCHAIN_PREFIX)$(CXX) -c -x c++-header $(CFLAGS) $(CXXFLAGS) -o $@ $<